There has been considerable effort devoted in recent years to solve various ecological and environmental problems such as air pollution, acid rain, etc. Combustion effluents and waste products from various sources are a major source of air pollution when discharged into the atmosphere. Unless the waste products are treated to remove deleterious components, the degradation of the environment will continue. Acid rain, forest and vegetation decline, changes in the ozone layer, harmful and irritating smog, etc., are examples of the results of the pollution of the atmosphere.
The common sources of pollution include internal combustion engines, industrial plants, utility boilers, gas turbines, and commercial establishments such as service stations, dry cleaners, etc. It has been estimated that power plants are responsible for about 1/3 of the annual NO.sub.x emissions while mobile sources such as automobiles and trucks are responsible for about 40% to about 50%. The types of air pollutants generated by such facilities include particulate emissions such as coal ash, sulphur compounds such as SO.sub.2 and SO.sub.3, carbon monoxide, ozone, and nitrogen oxides, commonly referred to collectively as "NO.sub.x ". One of the common components found in polluted air is nitrogen dioxide (NO.sub.2) which is known to be an extremely poisonous material. Nitrogen dioxide is introduced into the atmosphere from the various sources such as commercial plants producing nitric acid, but a major source of nitrogen dioxide is from nitric oxide (NO) formed by combustion processes of the types described above. The nitrogen oxide is formed during such combustion processes by (1) the reaction of nitrogen with atmospheric oxygen in the high temperature portion of the flame ("thermal fixation"); and (2) the oxidation of organic nitrogen compounds in the fuel on burning. The nitric oxide formed on combustion is converted to nitrogen dioxide on contact with air in the atmosphere.
Various procedures have been suggested to remove the oxides of nitrogen from waste gases so that the gases may be discharged into the atmosphere without harm to the environment. Nitrous oxides emissions from boilers, gas turbines and internal combustion engines have been reduced by modifying the design of the engine or boiler to be more efficient or to operate at a lower temperature. Other proposals for reducing nitrogen oxide emissions involve use of various chemicals to reduce the nitrogen oxide content of effluent gases by converting the nitrogen oxides to innocuous gases. Such chemical processes, however, generally require extremely high temperatures such as in the range of about 1600.degree. to about 2000.degree. F. and higher. The temperatures of some of these chemical reactions for reducing nitrogen oxide content have been reduced by utilizing catalysts which are effective in promoting the reduction of nitrogen oxide, but using a catalyst has certain disadvantages such as the expense of the catalyst, the life of the catalyst, the expense and difficulty of contacting the combustion effluents with the catalyst, etc. Accordingly, there has been continued emphasis on procedures for reducing nitrogen oxide emissions which do not involve the direct use of catalysts. Various techniques for reducing NO.sub.x emissions from various combustion processes are described in the article entitled "Reducing NO.sub.x Emissions," Power September 1988, pp S-1 to S-13.
Among the chemicals which have been suggested as being useful in reducing the nitrogen oxide content of combustion effluents are nitrogen-containing compounds such as ammonia, urea, cyanuric acid, etc. For example, U.S. Pat. Nos. 3,900,554; 4,335,084; 4,743,436; 4,849,192; and 4,851,201 describe processes utilizing ammonia to reduce nitrogen oxide emissions.
U.S. Pat. No. 3,867,507 describes a method for treating a stream of combustion effluents containing NO.sub.x and other contaminants wherein the stream is contacted initially with a hydrocarbon in the presence of oxygen to reduce the NO.sub.x to molecular nitrogen, and thereafter contacting the reduced stream with oxygen to oxidize all other contaminants to an oxidized state. The patentees indicate that any hydrocarbon, as well as oxygen-containing hydrocarbons, sulfur-containing hydrocarbons, nitrogen-containing hydrocarbons, etc. which yield carbon containing free radicals on pyrolysis can be used. Among the oxygenated hydrocarbons disclosed are alcohols, ethers, acids, ketones and aldehydes. A critical feature of the process is the ratio of total number of carbon atoms in the hydrocarbon per mole of NO.sub.x. The number of carbons in the hydrocarbon may be from 1 to 12. The second critical feature is the [O.sub.2 ]/[C]. The patentees further state that any carbon atoms bound to oxygen in the hydrocarbon compounds are not considered in the carbon-to-oxygen ratio, and this requirement excludes formaldehyde. Japanese Patent Publication No. 54-46172 describes the method for removing nitrogen oxide from waste combustion gases by adding alcohols or aldehydes to the combustion gas to reduce the nitrogen oxide at a gas-phase state in the presence of oxygen. The alcohols include methyl alcohol, ethyl alcohol, propyl alcohols and butyl alcohols. The aldehydes include formaldehyde, acid aldehyde, etc. The authors indicate that the reaction between the aldehyde or alcohol and the NO.sub.x in the gas can be conducted at temperatures of 200.degree. C. to about 600.degree. C., and more preferably, from about 400.degree. C. to 600.degree. C.
Canadian Patent 654,427 describes a process for removal of nitrogen oxides from SO.sub.2 and/or SO.sub.3 -containing gases. The process involves spraying water containing reducing agents into the gases to be purified. Formaldehyde is disclosed as one example of a useful reducing agent. The patentees indicate that the process also is useful for removing nitrogen oxides from gases which do not contain SO.sub.2 and/or SO.sub.3, but in this case SO.sub.3 is added to the gas while the gas is sprayed with dilute sulfuric acid containing reducing compounds.
Addition of oxygen-containing hydrocarbons (alcohols, aldehydes, ketones, acids, ethers, etc.) and/or their precursors for controlling nitrogen oxides in exhaust gases by conversion of NO to NO.sub.2 is described in U.S. Pat. No. 4,350,669; U.K. Patent 1,572,118 and Japanese Patent Application Nos. 52-14619; 52-42461; 53-76968; 53-128023; and 54-38268. U.S. Pat. No. 4,350,669 describes methanol and formaldehyde as useful oxygen-containing hydrocarbons, and methane, ethane and propane as examples of hydrocarbons.
U.S. Pat. No. 4,256,711 describes catalysts for decomposing nitrogen oxides present in gas streams. The catalysts include the borides, carbides, nitrides and silicides of metals such as Ti, V, Zr, Hf, etc.